Shear zones are zones of highly strained rock that form under brittle, ductile, or intermediate conditions. They record a history of deformation and can indicate the sense and amount of displacement. There are several types of shear zones defined by the dominant deformation mechanism (brittle, ductile, semibrittle, brittle-ductile). Determining the sense of shear is important and can be achieved through studying offset markers, foliation patterns, shear bands, inclusion shapes, and other indicators exposed in the shear zone.

1. Diego Torromé
Jorge Sevil
Urbez Majarena

2. 1.- The nature of shear zones.
• Shear zone is a zone composed of rocks that are more highly
strained than rocks adjacent to the zone.
• The intensity which rock can be deformed in shear zones is
astonishing (e.g. granites that seems schist).
• Shear zone can be formed under three main conditions:
 Brittle conditions: Generates a fault zone.
 Ductile conditions: (deformation +
metamorphism) Foliation, lineations,
folds….
 Intermediate conditions.
Fault in brittle conditions.Vadiello (Huesca).
• Shear zones provide a detailed record of the history of
deformation and permit us to determine the amount of strain, and
the sense and amount of displacement.

3. 1.1.- General characteristics.
• The distribution of strain in shear zones it´s mainly heterogeneous.
• There are a spatial gradient in the amount of strain. The strain is
higher in the center of the zone.
• There are two types of shear zones in order to their continuity:
 Continuous: Gradual increase of strain,
within break, ductile conditions.
 Discontinuous: Abruptt increase of strain,
sharp physical breaks, brittle conditions.
• The scale of length and displacement can
be very different magnitude (mm-km).
• Usually shear zones are much longer than thicker.
• Whether a shear zone appears continuous or
discontinuous depends on the scale at which we observe
the structures too.

4. 1.2.- Geometries.
• Subparallel margins
• Retain his thickness (over much of their length) but:
 Where the margins diverge, widening occurs, (specially in the
ends of a shear zone).
 Where the margins converge, icreases deformation and thinning
takes place.
• Commonly arranged in sets that may: Deflect toward one
another and link up or crosscut and displace one other.
• Normally planar to gently curved. Some can have complex
geometries. The curviplanar geometry may be:
 An original planar shear zone that was folded.
 An original curviplanar geometry, encompassing around more
rigid, less deformed object (e.g. pluton on a reginal scale, or
pebbles of a strong rock in a small scaller)

5. 1.3.- Offset and Deflection of Markers.
• They are generated when a shear zone cuts across a
preexisting feature (A compositional layering, a dike…)
• A marker cut by:
 A continuos shear zone -> Deflection in its
orientation and change in thickness.
 A discontinuos shear zone -> Offset across
the zone.
• Shear zones types according with the
relative displacement of rocks on
opposite sides.
 Strike-slip (dextral or sinistral)  Specifing which way the
 Normal hanging wall moved.
 Reverse (e.g. top to the west)
 Thrust  Vertical shear zones ->
 Oblique-slip (strike-slip + dip slip) e.g. west-side up.

6. 1.4.-Tectonic setting.
Shear zones form in plate boundaries of all types.
• Strike-slip zones as the San Andreas
fault of California.
• Plate convergence and crustal
shortening commonly have thrust and
reverse shear zones as Alpine-Zagros-
Himalayan belt. Some zones represent
the deep-level and others form beneath
basement-cored uplifts.
• Plate divergence and crustal extension.
Specially in regions of continental
rifting, such as the African rift.

7. 2. Types of shear zones
We can subdivide shear zones in four mainly types, based on the
characteristic type of deformation of each one:
1. A brittle shear zone contains fractures and other features formed by
brittle deformation.
2. A ductile shear zone displays structures that have been formed
shearing by ductile flow.
3. Semibrittle shear zones involves mechanisms such as pressure
solution and cataclastic flow.
4. The last one, brittle-ductile shear zones, shows evidence for both
brittle and ductile deformation.

8. 2.1. Brittle shear zones
• Brittle shear zones form in the upper part of the
crust, where the brittle deformation dominates.
• Shear zones formed in this conditions are
characterized by closely spaced faults, numerous
joints and shear fractures.
• These zones of intensely fractured and crushed
rocks associated with faults vary in thickness from
less than a mm to a km or more.
• The wall rocks outside a brittle shear zone may be
unaffected by the faulting, or may show a zone of
drag folding flanking the zone.

9. 2.2. Ductile shear zones
• Ductile shear zones are formed by shearing under
ductile conditions, in this case it produces at the
middle-lower part of the crust. Accordingly, we
generally see ductile shear zones with rocks we would
expect to find in the middle crust and deeper like
gneiss, schist, marble, migmatite, pegmatite…
• The principal feature of a ductile shear zone is that it
doesn’t display any physical break. Instead, differential
translation of rock bodies is achieved entirely by ductile
flow.
• Some rock types that form in ductile shear zones are
different from the normal metamorphic rocks. Such
rocks are called tectonites (we must indicate the type of
rock, e.g., marble tectonite).

10. 2.3. Semibrittle shear zones
• These zones are dominated by brittle deformation
mechanisms but contain some ductile aspects as well.
• Shear zones defined by en echelon folds can be either
semibrittle or ductile, depending of the conditions
under which they form. Many zones of en echelon folds
are associated with faults and are probably best
classified as semibrittle shear zones. The faults are
brittle features, but the folding may occur by ductile
mechanisms, such as pressure solution, without loss of
cohesion of the rocks.
Note: the term “en echelon” refers to closely-spaced, parallel or
subparallel minor structural features in rock that are oblique
to the overall structural trend.

11. 2.4. Brittle-Ductile shear zones
• Brittle-ductile shear zones contain evidence of
deformation by both brittle and ductile mechanisms.
• Brittle-ductile shear zones can be formed when:
1. the physical conditions permit brittle and ductile
deformation to occur at the same time
2. different parts of a rock have different mechanical
properties
3. a shear zone strains harden
4. physical conditions change systematically during
deformations
5. a shear zone is reactivated under physical conditions
different from those in which the shear zone
originally formed.

12. 3.Determining sense of shear
• One of the main goals in studying shear zones is to determine
the sense of shear (the direction one side of a shear zone is
displaced laterally relative to the other side).
• Shear-sense indicators are those features that show the sense
of shear for a deformation.
• Offset Markers.
• Foliation Patterns.
• Shear Bands, S-C Fabrics, Oblique
Micorscopic Foliation.
• Mica Fish.
• Inclusions.
• Pressure Shadows.
• Porphyroclasts and Porphyroblasts.
• Foliation Fish.
• Fractured and Offset Grains.
• Veins.
• Folds.

13. 3.1. Offset Markers
• We can determine both the amount
and sense of displacement if we
realize the similar-appearing
features on opposite side of the
shear zone were originally
continuous.
3.2. Foliation Patterns
• Systematic variations in the orientation
of foliation are common in ductile shear
zones and provide one of the most useful
shear-sense indicators.
• Foliation will be rotated towards
parellelism with the shear zones where
the rocks are more strongly deformed,
and the strain is higher.

14. 3.3. Shear Bands, S-C Fabrics, and Oblique
Microscopic Foliation.
• Shear bands are thin zones of very
high shear strain within the main
shear zone. A shear band is synthetic
if it is inclined in the same direction
as the overall sense of shear, and
antithetic if it is inclined in the
opposite direction.
• S-C fabrics. They consist of two
sets of planes: foliation planes (S-
surfaces) and shear bands (C-
surfaces). The clearest examples
are in mylonitic granitic and
gneissic rock.

15. • The oblique microscopic foliation is defined by aligned
subgrains oblique to the long axis of larger individual grains
and ribbons. The oblique foliation leans over in the direction
of shear relative to the main foliation defined by the larger
grains.

16. 3.4. Mica Fish 3.5. Inclusions
• They are commonly aligned • Inclusions provide sense-
with S-surfaces in S-C of-shear information by
fabrics, and lean over in the way they
the sense of shear. rotate, deform, recrystalliz
e, and interact with their
matrix.

17. 3.6. Pressure Shadows
• When inclusions are strong compared to the matrix, they
help shielding the matrix on the flanks of the inclusion from
strain. These shielded areas (pressure shadows) are
composed of less deformed matrix or of minerals that grew
or recrystallized during deformation.
3.7. Porphyroclasts and Porphyroblasts
• Rigid grains of one mineral within a more
strongly deformed matrix having a different
mineralogy. Knowing the shape due to
noncoaxial deformation, we can define the
sense of shear.

18. 3.8. Fractured and Offset Grains
• Porphyroclasts and other rigid inclusions may accommodate
deformation by becoming sliced up by small-scale or grain-scale
faults.
• The sense of displacement on such faults can be a shear-sense
indicator, but not a totally reliable one.

19. 3.9. Veins
• Most shear zone-related veins contain quartz and calcite. These
minerals are deposited from the fluids that filled the opened
fractures.
• Most veins form “perpendicular” to the axis of maximum
extension, because this is the direction in which tension
fractures form.